The objectives of this proposal are to develop a high-performance photon-counting 3-dimensional imager (2 spatial, 1 temporal) for biology and new methodologies and protocols taking advantage of its new capabilities. This detector will be optimized for ultrahigh sensitivity fluorescence spectroscopy in the visible and near-infrared spectral range (500-850 nm). It will have: (i) a high detection efficiency based on a fast GaAs photocathode: ~35 %, (ii) will register single-photons with high spatial resolution using a cross-strip anode design: 35 um, and (iii) high temporal resolution using microchannel plates: 250 ps, and (iv) be capable of both high local and global couting rates (100 kHz, 20 MHz). These characteristics are condensed in the acronym "H33D" (pronounced heed) for High-spatial, High-temporal resolution, and High throughput (the 3 "H'"s, hence H3) 3-Dimensional detector. The H33D detector will be suitable for a variety of biomedical applications, such as imaging and spectroscopy of single molecules, molecular complexes and macromolecules in living cells and tissues, as well as in vivo animal imaging, some of which will be studied during the proposed 4 year span of this research proposal. The last application will be a first step towards using the H33D detector for biomedical imaging of fluorescent molecular probes in human patients with unprecedented sensitivity. These characteristics of our interdisciplinary research proposal regrouping scientists with a biophysical/biochemical background and scientists with a physics/instrumentation background are in perfect match with the vision expressed during the Biomedical Imaging Symposium 'Visualizing the Future of Biology and Medicine" organized by the BECON in 1999, and fits particularly well within the mission of the NIGMS and the NIBIB, who are part of BRG program announcement PA-02-011. Relevance of this research to public health: The richer information provided by the H33D detector will lead to a better understanding of cellular processes at the molecular level, a prerequisite step to design intelligent counter-measures aimed at curing abnormal cellular behavior with medical compounds. The long-term application of the H33D detector for live animal and human imaging will provide a much needed high- sensitivity instrument for diagnosing and monitoring the evolution of diseases at the molecular level.